This invention relates to the production of ethylene glycol, and more particularly, to a process for production of ethylene glycol wherein levels of by-products are reduced.
Ethylene glycol is a well-known chemical useful in a number of commercial applications such as preparation of antifreeze compositions and certain polyesters.
A variety of processes for production of ethylene glycol has been proposed. For example, U.S. Pat. No. 1,982,545 (Skarblom) discloses a process comprising contacting ethylene, oxygen and water in the presence of an iodine-containing catalyst such that the following reactions occur: EQU C.sub.2 H.sub.4 +I.sub.2 +H.sub.2 O.fwdarw.CH.sub.2 OHCH.sub.2 I+HI EQU CH.sub.2 OHCH.sub.2 I+H.sub.2 O.fwdarw.CH.sub.2 OHCH.sub.2 OH+HI EQU 2HI+1/2O.sub.2 .fwdarw.I.sub.2 +H.sub.2 O
The net reaction is as follows: ##STR1## According to Skarblom, reaction conditions such as temperature, pressure and reactant concentrations and partial pressures are not critical except to the extent that they affect reaction rates.
Despite the apparent simplicity of the process according to Skarblom, the same is not entirely satisfactory, in part, because undesirably high levels of by-products typically are produced. Such by-products include acetaldehyde and condensed glycols such as diethylene glycol and p-dioxane. In a sense, formation of condensed glycols does not constitute a loss of reactants because these by-products equilibrate with the ethylene glycol. Acetaldehyde, on the other hand, does not undergo such equilibration, and accordingly, its production does result in a loss of reactants and decreased yield of the desired product.
Methods to reduce by-product formation in the Skarblom process have met with varying degrees of success. U.S. Pat. No. 3,928,474 (Witheford) discloses a process for production of ethylene glycol wherein the iodine-catalyzed reaction of ethylene, oxygen and water is conducted under superatmospheric pressure after which a distillation step is conducted at subatmospheric pressure. According to a further aspect of Witheford, the iodine-catalyzed reaction of ethylene, oxygen and water is conducted under superatmospheric pressure in two separate reaction zones, one containing an ethylene-rich feed and the other containing an oxygen-rich feed, followed by distillation under subatmospheric pressure such that condensed glycol formation is reduced and free iodine and organoiodine by-products are removed from the ethylene glycol in the distillation step. In Witheford's examples, preparation of ethylene glycol in 75% yield based upon consumed ethylene is illustrated. The yield of by-product condensed glycols was 8% (7% diethylene glycol and 1% triethylene glycol).
Although the process according to Witheford is reported to give reduced levels of condensed glycols as compared to the Skarblom process, the former is disadvantageous in that additional equipment is required. Further, Witheford does not provide for limiting formation of acetaldehyde or removal thereof. This deficiency is discussed in greater detail in U.S. Pat. No. 4,045,500 (Onsager et al.) which teaches that by-product acetaldehyde from the Skarblom or Witheford processes is inherently recycled to the reaction zone and converted to acetic acid which reacts with ethylene glycol and higher glycols to form acetates which are difficult to remove from the desired product. According to Onsager et al., this difficulty is remedied by separating the product obtained by the iodine-catalyzed reaction of ethylene, oxygen and water into a low-boiling fraction containing acetaldehyde, an ethylene glycol fraction and a high-boiling fraction containing higher glycols. The ethylene glycol fraction is removed for purification and acetaldehyde is separated from the low-boiling fraction. The low-boiling fraction remaining after separation as well as the higher-boiling fraction are recycled to the reaction zone. Ethylene glycol and higher glycols equilibrate in the reaction zone with the result that after a certain level of higher glycols has built up, further generation of such materials is suppressed.
While Onsager et al. does provide for removal of by-product acetaldehyde from ethylene glycol, there is no attempt to limit production of the by-product. Thus, raw materials otherwise convertible to the desired product are lost. Further, Onsager et al. requires handling of large amounts of higher glycols which can be disadvantageous from the standpoint of process efficiency.
From the foregoing it can be appreciated that it would be desirable to provide an improved process for production of ethylene glycol wherein the abovedescribed problems are reduced or eliminated. It is an object of this invention to provide an improved, iodine-catalyzed process for production of ethylene glycol. A further object is to provide a process wherein generation of by-product acetaldehyde is retarded. A further object is to provide a process in which generation of both acetaldehyde and condensed glycols is retarded. A still further object of the invention is to provide a process in which high selectivity to ethylene glycol is attained with reduced loss of raw materials through by-product formation. Other objects of the invention will be apparent to persons skilled in the art from the following description and the appended claims.
I have now found that the objects of this invention can be attained by contacting a nonflammable feed comprising ethylene, oxygen and water with a catalyst comprising iodine under reaction conditions in a reaction zone in which there is maintained a relatively low partial pressure of ethylene. Surprisingly, maintenance of low ethylene partial pressures in the reaction zone results in decreased formation of acetaldehyde, and accordingly, reduced loss of raw materials through by-product formation. As a result, selectivity to ethylene glycol is increased. This is in contrast to the aforesaid patents which suggest that ethylene partial pressure is important only from the standpoint of reaction rate. While ethylene partial pressure does influence reaction rate, the large values proposed and preferred by the prior art are, in fact, detrimental to the attainment of maximum selectivity to ethylene glycol.
According to a further aspect of this invention, formation of condensed glycol by-products is retarded by maintaining a relatively low level of ethylene glycol in the reaction zone. The relationship between ethylene glycol levels and condensed glycol formation, while not a directly proportional one, is such that prompt removal of ethylene glycol can result in substantial reductions in formation of condensation products thereof. This, of course, leads to greater selectivity to ethylene glycol and there is reduced need for costly by-product removal operations.